Mechanisms for spreading particulate matter over a ground surface are well known and in common use. Material spreaders can be classified in two general categories: broadcast spreaders and drop spreaders. Drop spreaders discharge material directly downwardly on a soil or other type of surface from a series of apertures in a container structure that holds the material to be spread. Typically as the containment structure is propelled along the ground, a mechanical member disposed above the apertures moves in coordinated relation with the rotation of the wheels on the spreader to sift and meter the material through the apertures for even distribution on the ground. Typically the apertures and the mechanical member extend the transverse width of the containment structure.
Broadcast spreaders typically disperse particles outwardly from a containment structure by dropping the same from a plurality of large apertures in the containment structure onto a rapidly rotating structure rotating in a plane generally parallel to the ground. More particularly, in a typical walk-behind spreader, the apertures are positioned to drop the material on a rearward portion of the rotating structure so that the material is thrown outwardly generally forwardly and to the sides of the container structure and is not thrown rearwardly on the spreader user. The rotating structure is constructed and arranged to throw the particles outwardly from the spreader over an area wider than the width of the spreader.
The drop spreaders and broadcast spreaders offer the user certain advantages and disadvantages. Uniform application of material is critical for certain pesticides and some herbicides and drop spreaders spread particulate matter in a highly uniform pattern over a path the transverse width of which is essentially the width of the spreader. The uniformity of distribution afforded by drop spreaders does not depend on particle size or density, the travel speed of the spreader or, because drop spreaders are typically low to the ground, wind conditions.
Drop spreaders have several disadvantages, however. Because the path width of the applied material is limited to the width of the spreader, drop spreaders are typically slower in covering a ground area of a given size with material compared to the broadcast spreaders. Because the path width is well defined, gaps and/or overlapping between adjacent paths can result unless the drop spreader is carefully navigated along a path immediately adjacent a previously traversed path. Gaps and overlapping may be undesirable, depending on the material being applied. The low ground clearance of the drop spreader may present difficulties when attempting to traverse terrain with high vegetative ground cover or abrupt changes in ground contour. Drop spreaders are also sometimes hard to propel, particularly when the containment structure is filled to capacity, because the mechanical member that sifts and meters the material through the apertures extends the width of the material.
Broadcast spreaders offer advantages over drop spreaders in some situations. Because they cast the particulate matter outwardly over a path wider than the width of the spreader, broadcast spreaders cover a ground area with the particulate matter of give size more quickly than a drop spreader of comparable size and capacity. Broadcast spreaders typically push relatively easily and because they have only a few large apertures in a rearward central part of the containment structure, these apertures usually remain relatively free of clogs.
Broadcast spreaders spread material with a non-uniform distribution pattern, however. More specifically, typically the central portion of the pattern has a relatively high particle density and the parallel transverse edges (transverse to the direction of travel of the spreader) have a relatively low particle density. This characteristic of the broadcast spreader can be an advantage or a disadvantage, depending on the type of material being spread and the objectives of the spreader user.
If a typical fertilizer is being spread on a lawn or rock salt is being applied to an icy driveway, for example, the uniformity of particle density is not critical and a broadcast spreader is well suited to the task. The lower particle density on the transverse edges can be an advantage in these applications because it is often desirable to have a degree of coverage overlap between adjacent paths to prevent gaps in coverage and the relatively low peripheral density allows a degree of overlap while preventing over application of the material in the overlapping area. Non-uniform transverse particle density is frequently unwanted when applying pesticides or herbicides, however, for reasons cited above, hence the drop spreader is preferred in these applications, despite the aforementioned disadvantages of the same.
A material spreader disclosed in U.S. Pat. No. 4,032,074 to Amerine provides a rotary broadcast type spreader that includes deflection means for deflecting the broadcast material downwardly to form a uniform pattern of controlled configuration. The preferred deflection means is in the form of a shroud the horizontal cross-section of which is in the shape of an ellipse. An impeller on the material spreader sends the material outwardly 360 degrees into contact with the interior of the shroud which deflects the material onto the ground in a uniform pattern. Because the shroud is present, asymmetrical discharge of the particulate matter onto the impeller is not required or provided by the material spreader because the shroud blocks particulate matter from hitting a user walking behind the material spreader. The '074 material spreader is not constructed to be reconfigured to function as a conventional broadcast type spreader and therefore has limited utility. A need exists for a material spreader that is convertible between broadcast and drop modes and so provides the advantages of both types of spreaders and eliminates the disadvantages of each.